How to save (not as a blender file)
You want to export your model, rather than save the file.

You need to know which file format your 3d viewer uses and select it from the menu.  Not all of the export formats are enabled by default.  You need to go to user preferences (CTRL-ALT-U), switch to the addons tab and look for the right exporter

You can find most of the exporters by selecting the import-export category.

If the exporter you want isn't there, then search for the question 'how do I export files of type